In order to study the osteogenic action of hydroxylapatite (HA) on the dental pulp, a pulp capping experiment was designed using the rat upper molar. Under general anesthesia, molar teeth in 14 male Sprague-Dawley rats were pulp capped with Osteogen (HA) or with Dycal as a control material. After pulp capping, the maxillary molars cavities were restored with amalgam and a pedodontic steel crown was adjusted and sealed over the molar teeth on either side of the maxilla. After 7 days, the areas of necrosis and acute inflammation were more evident in the pulps treated with Dycal than with Osteogen. Hard tissue formation began to appear around dentinal chips in the pulp and extended from the cavity walls into the pulp regardless of the material that was used. Furthermore, this calcified material was scattered throughout the pulp when Osteogen was used, but was not observed in the Dycal treated pulps. The hard tissue formation was thought to be due to the putative fibroblasts and odontoblasts found in the pulp. After 28 days dense dentinal tissue was observed bridging the exposure site when Dycal was used. The dentinal tissues formed with Osteogen was always of a globular type, and showed an irregular distribution. Since Osteogen tends to cause areas of dystrophic calcification in the pulp, its use is not be recommended for pulp capping purposes in humans, because these areas of calcification would make future endodontic treatment difficult.